Advanced topic 9, 2012
User-Centric Management of Wireless LANs The content of this wiki page was adapted from the paper above. All credit goes to original authors. Topic 9 from Advanced Topics 2012 Overview of NM in WLANs Basic management entities and activities There are three mian management entities defined in IEEE 802.11 family standards: *Entities: ** Station management (SME) ** MAC Sub-layer Management Entity (MLME) **Physical Layer Management Entity (PLME) The dominant functions of MLME and PLME are to in-phase, managing power, combination and recombination, which is usually used to process latency in network device firmware, being more suitable to be executed in hardware instead of network management. SEM is to gather the inter-layers status and to configure specific parameters.It is the central entity in network management.Meanwhile, MLME and PLME provide some system status to SME by interfaces between MLME and SME. There are also three activities of APs: *Activities: **Parameter configuration **Users information management **Network monitoring Specifically, parameter configuration includes channel selection and power configuration; user information management contains AP association lists and management; and network monitoring relates to both downlink and uplink monitoring traffic from users to APs. Moreover, there are two types among those activities. Passive activities include the priority management and the network monitoring while positive activities include the parameter configuration and the AP association lists. Additionally, during network deployment, power configuration can be set without much change to it.Therefore, the scope of AP association and channel selection is critical when it comes to most parts of network management. AP Association and Channel Selection Firstly, see the Fig. 1 showing the basic working flow over the network management framework in traditional WLANs. AS can be seen from the figure, an AP needs to select a channel either manually or via the configuration of neighbouring APs within sensing coverage. Then, configuration information will be sent in beacon frames to those users nearby. However, the user is more likely to only select the AP from those access points with enough signal power even if this user can sense and know the signal power in all the transmissions within sensing coverage. In the last step, the AP adds the user into its serving list based on whether the user is admitted with some security credentials like the permissible MAC address. Furthermore, the AP association and channel selection don’t use AP-related or user-related information so they tend to be AP-centric. User-centric network management framework The fundamental idea of user-centric network management framework, instead of traditional AP-centric network management framework, is to involve the users participation in network management framework. This framework can solve the throughput problems AP-association left. The basic principle is to provide the pipeline information exchange platform and duplexing selection between APs and users that enable to enhance traditional APs one-side control to consider what user needs. The detailed user-centric network management working procedures illustrate below. First of all, AP selects a channel manually or based on neighboring APs’ sensing range. After that, AP delivers configuration information (like channel, data rate, and MAC address) periodically to users by beacon within its sensing range. The user, collecting the APs status within its receiving range periodically, considers the network conditions (like beacon frame transmission and data transmission) which is also as network snapshot and piggybacks the network snapshot, access priority and association request frame to all candidate APs in its receiving range. AP, which receives the frame, calculates the potential throughput for users by users provided information and transmission status by itself and returns the response frame with calculated results to the users by piggyback. Then user will choose the AP that provides the maximum potential throughput and inform it with a new association request. The selected AP replies with a response frame to finalize the association and stores the users’ network snapshot. It is worthy mentioning that AP not only can provide the potential throughput and be selected by users but also can dynamically select the operational channel and optimize the aggregate throughput by the collected user information and snapshot under user-centric network management system. Moreover, users can select the AP that can provide the maximum throughput, avoiding being passive selected by AP. PART 5 Implementing the Management Framework 1.The dual-threshold triggering conditions A new association between AP and user triggered by users only when the existing association throughput is lower than a lower-bound threshold {C} or a new (optimal) AP-association potential throughput is much stronger than its current throughput by at least a minimum improvement threshold {C}. Specifically, The user {C} executes a new AP association procedure only if 0< {C}< min . A new association triggered by AP happens only when the current aggregate downlink throughput is lower than a lower-bound threshold {C} or a new (optimal) channel selection potential throughput is much stronger than its current throughput by at least a minimum improvement threshold {C}. Specifically, AP {C} establishes a new AP association procedure only if: 0< {C}< min . 2. The dual period operation AP association or channel selection should be taken periodically to constantly optimize to such operation. Then, select a comparatively short T in order to have a initialization in the process, for each Ta, users who have been connected to the AP need to recalculate the throughput that may be potentials to them. Since the channel selection operation will lead to the reconnection of all connected users, so select a longer decision period Tc rather than the shorter period Ta, like the previous procedure, the AP assesses its general performance and determines whether to set a new channel selection. At last, �� ∈ (0, 1) and �� ∈ (0, 1) acted as the smoothing factors depending on the realistic considerations can be introduced to get the average of traffic ups and downs: �� and �� normally are set to 0.9. Additionally, ������, and ������,��+���� stand for updated throughput o f ���� at time t and average throughout in time range [��, �� + ����) respectively, which can be used to define ����,�� and ����,��+���� as well. The throughput of the APs and the connected users should be updated at each instant decision time and see if the triggering conditions are satisfied, then the finest channel selection described above can be used. However, even though short update intervals can result in more throughput, hard breakdowns caused from each recombining period in transmission will affect users because of the high frequency in recombining. The effect on hard breakdowns has to be assessed through specifying a WLAN APs, in order to analyze the simulated deployment and then get the value for ���� = 600s and ���� = 1800s. Obviously, the lower-bound can be set at following since the choice of it is in linear proportion to the average throughput: So, it can be seen that the calculation depends on the link transmission speed ® and the number of users (Nu). And CR∈ (0, 1) is a mapping coefficient, while NA is the number of APs. For example, if R = 54 Mbps, then set CR = 0.5. Meanwhile, thresholds are improved aimed at enlarging the throughput and the choice of the enhanced thresholds will be determined by the period time of breakdown. Here, set these thresholds as follows for later simulations: Where Tb is the period time of breakdown and C stands for a constant for deciding the improvement extent which is always larger than 1 PART 6 Simulations In order to analyze the throughput performance of our purposed user-centric network management framework, the author uses the software NS2 to do the simulations. The parameters can be seen blow : All the APs and users are randomly and uniformly distributed with density 1AP per unit area , while the user density varies with the simulation senarios. A : Effect of Access Priority This senario is : one AP serves 8 users. The downlink throughput is measured of one user in different priorities. The available through of the user with high access priority is always greater than that with low access priority. And the available throughput for the considered user decreases with the number of users with high priority no matter what access priority it has. Access priority distribution in the AP affects the potential through, and since the throughput decreases as the number of users increases,the AP association can play a vital role in increasing the throughput for a user. B : AP Association over User-centric Network Management Framework This case is: there are always data packets to transmit on all the queues of all APs. One user is “tagged” as the observing user and we fix the AP association of all other users. We evaluated three types of management frameworks: one is AP selection under the traditional management framework, the second is optimal selection in AP-centric framework and the last is user-centric management framework. It is obversed that AP association based on the usercentric network management framework always outperforms other frameworks, and the throughput gain compared to ranges from 25''.0% to 90.5%. The throughput comes from access priority and the network conditon. It's possible that the current AP has a lot of users with high priority. Then, a new AP association with less high priority users will probide better throughput for that uses. Again, if the network condition of the current AP is poor(e.g. high noise or fading) resulting in poor throughput, the throughput can be increased by associating with a new AP with better network condition. C :Channel Selection over User-centric Network Management Framework Next, we investigate the effect of the channel selection on the total throughput of users associated with one AP. We fix the user density to 2users per unit area area, and take one AP as observing AP which associates with two observing users. User-centric management provides highest throughput -max 69.3%, while as the APs get saturated with load, the throughput becomes similar for all the system. * Channel selection in user-centric management framework is initiated by the AP. If the channel condition is poor, than the AP decides to change the channel to a potentially better channel. The channel can give poor throughput for several reason-channel selective fading of channel collision with neighboring AP are some of the reason. We can see that we can get more throughput from user-centric management, with a maximum gain of 69.3%. However, as the APs get saturated with users, the throughput performances for all the management systems become similar as all the paths are full. D : Impact of Dual-threshold in Implementation Finally, we explore the impact of dual-threshold for triggering management operations on user throughput. we get that the value of the dual-threshold depends on the setting of ���� and ��. We have the same simulationscenario as Section VI-B to explore the impact of dualthreshold on user throughput. The user density is fixed to 2 users per unit area, and the target user is configured as highpriority user. Here, CR∈ (0, ''1) is a mapping factor for available lower-bound throughput and physical transmitting speed C>1 is a constant for leveraging the improvement The throughput increases as CR increases. This happens because if the lower bound threshold is small it won't trigger the management operation. So throughput will not increase. AND, the throughput decreases as C increases. This is because a large value of minimum improvement threshold will also hinder the execution of management operation. * *